1. Field of the Invention
The invention relates generally to long-haul optical fiber networks, more particularly, to long-haul spans comprising dispersion managed optical fiber with signal amplification.
2. Background of the Invention
Signal degradation encountered when transmitting optical signals over long-haul optical fiber has greatly increased the need for improved optical signal amplification devices along the transmission path. Specifically, long-haul optical signal amplification presently suffers from amplification of noise along with the optical signal, resulting in a degraded signal to noise ratio (SNR) at the receiving node.
Presently, one method of long-haul signal amplification is achieved by utilizing a Raman amplification scheme. Raman amplification utilizes a pump laser optically coupled to the receiving node. The Raman pump laser provides an amplification signal propagating along the transmission path in a direction opposite the optical signal. As the amplification signal travels along the transmission path, energy is gradually transferred from the amplification signal to longer wavelengths of the optical signal through stimulated Raman scattering.
The power of the amplification signal is greatest near the output node of the long-haul optical transmission system where the pump laser inputs to the optical cable. Optical intensity of the amplification signal can be represented by the equation: Intensity=(Laser Light Power/Aeff), where Aeff is the effective cross sectional area of the fiber.
Negative dispersion fiber, and in particular negative dispersion negative slope (NDNS) optical fiber, or so-called slope compensating optical fiber (SCF), is used to compensate for the difference in dispersion and dispersion slope of one or more optical signals at one or more wavelengths transmitted in a positive dispersion single mode transmission fiber. NDNS optical fiber also typically has a small Aeff when compared to other forms of optical fiber used for optical signal transmission, such as typical single mode fiber which is used as the positive dispersion transmission fiber in 1550 nm systems. The small Aeff results in higher pump laser intensity which results in greater amplification of the transmitted optical signal.
Amplification signal power tends to degrade at an approximate rate of 0.25 dB per 1 km of NDNS optical fiber as it travels along the long-haul optical transmission system at 1550 nm. Further, the minimum absolute dispersion of a particular wavelength of all of the wavelengths is typically in the range of 0 to 300 ps/nm. NDNS optical fiber is utilized, at least partially, so that the difference between the absolute dispersion between the wavelengths is very small.
In conventional communication systems, the optical signal first enters single mode fiber and then propagates into NDNS fiber. A Raman pump laser is optically coupled to the output of the NDNS section.
In Raman amplification, not only is the desired input signal amplified, but ambient noise introduced by a variety of sources as the input signal travels along a section of optical fiber is also amplified, resulting in a degraded SNR at the receiving node. The ambient noise being amplified is at least partially generated by multi-path interference (MPI) from double-Rayleigh back-scattering (DRBS) and Rayleigh back-scattering of amplified spontaneous emission (ASE).
The above mentioned noise degradation is particularly a problem in small Aeff fiber such as NDNS fiber, primarily because most of the Raman gain occurs in the NDNS section of the long-haul optical transmission system. The small Aeff of NDNS fiber dramatically increases the fraction of Rayleigh back-scattering falling into its propagating mode. This leads to rapid growth of noise with increasing Raman gain. In addition, at higher Raman gain, the total amplified signal power at the fiber section output becomes comparable to that of the pump and causes depletion. This, in turn, substantially degrades the Raman noise figure (NF). The noise properties of distributed Raman amplifier are typically characterized by using a “lumped noise figure” which is defined as a noise figure of an equivalent lumped amplifier placed after the fiber span that produces the same gain and optical signal-to-noise ratio as the Raman amplifier in question. For a typical Raman amplifier with polarization-independent gain GR, which is the signal power at the amplifier input divided by the signal power at the amplifier output, such a lumped noise figure, hereinafter referred to as “noise figure” or “Raman noise figure”, satisfies the following equation:NF=[1+PASE/(hvBO)]/GR,where PASE is the power of amplified spontaneous emission (ASE) in two polarizations, generated by Raman amplifier within optical bandwidth BO centered at signal frequency v, and h=6.62×10−34 Joule*second is Planck's constant.